Overlapping functions of the cell adhesion molecules Nr-CAM and L1 in cerebellar granule cell development

Epithelial morphogenesis and intestinal cancer: new insights in signaling mechanisms

In this review, the major signal transduction pathways that have been shown to play an important role in intestinal homeostasis are highlighted. Each of them, the Wnt, Notch, Hedgehog, and Bone Morphogenetic Protein, as well as growth-factor regulated Receptor Tyrosine Kinases are depicted with a special emphasis through their involvement in stem cell maintenance and their role in intestinal tumorigenesis. Finally, we discuss recent data on the final steps of tumor progression, notably the formation of distant metastases. This multistep process is highly complex and still far from being understood while being of major importance for the survival of patients with digestive cancer.

Abnormal axonal guidance and brain anatomy in mouse mutants for the cell recognition molecules close homolog of L1 and NgCAM-related cell adhesion molecule

Cell recognition molecules of the L1 family serve important functions in the developing and the mature nervous system. Mutations in genes encoding the L1 family members close homolog of L1 (CHL1) and NgCAM-related cell adhesion molecule (NrCAM) have been found to alter connectivity and morphology of several brain regions. In order to emphasize similarities and differences of these two structurally related molecules, null mutants for CHL1 and NrCAM were directly compared with respect to axonal guidance in the hippocampus and the olfactory bulb and the sizes of the ventricular system and the cerebellar vermis using a combined structural magnetic resonance imaging (MRI) and histological approach. The results demonstrate that the absence of CHL1 leads to aberrant hippocampal mossy fiber projections whereas in both mutants, CHL1 and NrCAM, the guidance of the olfactory nerve projections is disturbed. Both mutations also alter the size of the ventricular system and the vermis with a specific profile of changes and partially opposite effects in each of the mutants. CHL1/NrCAM double-mutant mice do not show any enhancement of the single mutant's phenotype but balance the opposing effects on the ventricular system. In summary, the results show that CHL1 and NrCAM both affect axonal guidance and the anatomy of the ventricular system and the cerebellar vermis but act differently on these processes.

L1 and NCAM adhesion molecules as signaling coreceptors in neuronal migration and process outgrowth

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily engage in multiple neuronal interactions that influence cell migration, axonal and dendritic projection, and synaptic targeting. Their downstream signal transduction events specify whether a cell moves or projects axons and dendrites to targets in the brain. Many of the diverse functions of CAMs are brought about through homophilic and heterophilic interactions with other cell surface receptors. An emerging concept is that CAMs act as coreceptors to assist in intracellular signal transduction, and to provide cytoskeletal linkage necessary for cell and growth cone motility. Here we will focus on new discoveries that have revealed novel coreceptor functions for the best-understood CAMs--L1, CHL1, and NCAM--important for neuronal migration and axon guidance. We will also discuss how dysregulation of CAMs may also bear on neuropsychiatric disease and cancer.

Voltage-gated Na+ channel beta1 subunit-mediated neurite outgrowth requires Fyn kinase and contributes to postnatal CNS development in vivo

Voltage-gated Na(+) channel beta1 subunits are multifunctional, participating in channel modulation and cell adhesion in vitro. We previously demonstrated that beta1 promotes neurite outgrowth of cultured cerebellar granule neurons (CGNs) via homophilic adhesion. Both lipid raft-associated kinases and nonraft fibroblast growth factor (FGF) receptors are implicated in cell adhesion molecule-mediated neurite extension. In the present study, we reveal that beta1-mediated neurite outgrowth is abrogated in Fyn and contactin (Cntn) null CGNs. beta1 protein levels are unchanged in Fyn null brains, whereas levels are significantly reduced in Cntn null brain lysates. FGF or EGF (epidermal growth factor) receptor kinase inhibitors have no effect on beta1-mediated neurite extension. These results suggest that beta1-mediated neurite outgrowth occurs through a lipid raft signaling mechanism that requires the presence of both fyn kinase and contactin. In vivo, Scn1b null mice show defective CGN axon extension and fasciculation indicating that beta1 plays a role in cerebellar microorganization. In addition, we find that axonal pathfinding and fasciculation are abnormal in corticospinal tracts of Scn1b null mice consistent with the suggestion that beta1 may have widespread effects on postnatal neuronal development. These data are the first to demonstrate a cell-adhesive role for beta1 in vivo. We conclude that voltage-gated Na(+) channel beta1 subunits signal via multiple pathways on multiple timescales and play important roles in the postnatal development of the CNS.

Axonin-1/TAG-1 is required for pathfinding of granule cell axons in the developing cerebellum

BACKGROUND

Neural development consists of a series of steps, including neurogenesis, patterning, cell migration, axon guidance, and finally, synaptogenesis. Because all these steps proceed in a constantly changing environment, functional gene analyses during development have to take time into account. This is quite challenging, however, as loss-of-function approaches based on classic genetic tools do not allow for the precise temporal control that is required for developmental studies. Gene silencing by RNA interference (RNAi) in combination with the chicken embryo or with cultured embryos opens new possibilities for functional gene analysis in vivo. Axonin-1/TAG-1 is a cell adhesion molecule of the immunoglobulin superfamily with a well defined temporal and spatial expression pattern in the developing vertebrate nervous system. Axonin-1/TAG-1 was shown to promote neurite outgrowth in vitro and to be required for commissural and sensory axon pathfinding in vivo.

RESULTS

To knock down axonin-1 in a temporally and spatially controlled manner during development of the nervous system, we have combined RNAi with the accessibility of the chicken embryo even at late stages of development. Using ex ovo RNAi, we analyzed the function of axonin-1/TAG-1 in cerebellar development. Axonin-1 is expressed in postmitotic granule cells while they extend their processes, the parallel fibers. In the absence of axonin-1 these processes still extend but no longer in a parallel manner to each other or to the pial surface of the cerebellum.

CONCLUSION

Axonin-1/TAG-1 is required for the navigation, but not for the elongation, of granule cell processes in the developing cerebellum in vivo.

Neuronal cell adhesion molecule deletion induces a cognitive and behavioral phenotype reflective of impulsivity

Cell adhesion molecules, such as neuronal cell adhesion molecule (Nr-CAM), mediate cell-cell interactions in both the developing and mature nervous system. Neuronal cell adhesion molecule is believed to play a critical role in cell adhesion and migration, axonal growth, guidance, target recognition and synapse formation. Here, wild-type, heterozygous and Nr-CAM null mice were assessed on a battery of five learning tasks (Lashley maze, odor discrimination, passive avoidance, spatial water maze and fear conditioning) previously developed to characterize the general learning abilities of laboratory mice. Additionally, all animals were tested on 10 measures of sensory/motor function, emotionality and stress reactivity. We report that the Nr-CAM deletion had no impact on four of the learning tasks (fear conditioning, spatial water maze, Lashley maze and odor discrimination). However, Nr-CAM null mice exhibited impaired performance on a task that required animals to suppress movement (passive avoidance). Although Nr-CAM mutants expressed normal levels of general activity and body weights, they did exhibit an increased propensity to enter stressful areas of novel environments (the center of an open field and the lighted side of a dark/light box), exhibited higher sensitivity to pain (hot plate) and were more sensitive to the aversive effects of foot shock (shock-induced freezing). This behavioral phenotype suggests that Nr-CAM does not play a central role in the regulation of general cognitive abilities but may have a critical function in regulating impulsivity and possibly an animal's susceptibility to drug abuse and addiction.

NrCAM, a neuronal system cell-adhesion molecule, is induced in papillary thyroid carcinomas

NrCAM (neuron-glia-related cell-adhesion molecule) is primarily, although not solely, expressed in the nervous system. In the present study, NrCAM expression was analysed in a series (46) of papillary thyroid carcinomas (PTCs) and paired normal tissues (NT). Quantitative reverse transcriptase (QRT)-PCR revealed that NrCAM expression was upregulated in all PTCs compared to normal thyroid, whatever the stage or size of the primary tumour. NrCAM transcript levels were 1.3- to 30.7-fold higher in PTCs than in NT. Immunohistochemistry (IHC) confirmed that the expression of NrCAM was considerably higher in tumours (score 2+/3+) than in adjacent normal paratumoural thyroid tissue. The NrCAM protein was detected in all but three (93.3%) PTC samples, and it was mainly cytoplasmic; in some cases there was additional membranous localisation – basolateral and partly apical. In the normal thyroid and tissues surrounding tumours, focal NrCAM immunolabelling was seen only in follicles containing tall cells, where staining was restricted to the apical pole of thyrocytes. Western blot analysis corroborated the QRT–PCR and IHC results, showing higher NrCAM protein levels in PTCs than in paired NT. The level of overexpression of the NrCAM mRNA in tumourous tissue appeared to be independent of the primary tumour stage (pT) or the size of the PTC. These data provide the first evidence that NrCAM is overexpressed in human PTCs at the mRNA and protein levels, whatever the tumour stage. Thus, the induction and upregulation of NrCAM expression could be implicated in the pathogenesis and behaviour of papillary thyroid cancers.

Oligodendrocyte ablation affects the coordinated interaction between granule and Purkinje neurons during cerebellum development

Oligodendrocytes (OLs) are the glial cells of the central nervous system (CNS) classically known to be devoted to the formation of myelin sheaths around most axons of the vertebrate brain. We have addressed the role of these cells during cerebellar development, by ablating OLs in vivo. Previous analyses had indicated that OL ablation during the first six postnatal days results into a striking cerebellar phenotype, whose major features are a strong reduction of granule neurons and aberrant Purkinje cells development. These two cell types are highly interconnected during cerebellar development through the production of molecules that help their proliferation, differentiation and maintenance. In this article, we present data showing that OL ablation has major effects on the physiology of Purkinje (PC) and granule cells (GC). In particular, OL ablation results into a reduction of sonic hedgehog (Shh), Brain Derived Neurotrophic Factor (BDNF), and Reelin (Rln) expression. These results indicate that absence of OLs profoundly alters the normal cerebellar developmental program.

Neural recognition molecules of the immunoglobulin superfamily: signaling transducers of axon guidance and neuronal migration

Recognition molecules of the immunoglobulin superfamily have important roles in neuronal interactions during ontogeny, including migration, survival, axon guidance and synaptic targeting. Their downstream signal transduction events specify whether a cell changes its place of residence or projects axons and dendrites to targets in the brain, allowing the construction of a dynamic neural network. A wealth of recent discoveries shows that cell adhesion molecules interact with attractant and repellent guidance receptors to control growth cone and cell motility in a coordinate fashion. We focus on the best-studied subclasses, the neural cell adhesion molecule NCAM and the L1 family of adhesion molecules, which share important structural and functional features. We have chosen these paradigmatic molecules and their interactions with other recognition molecules as instructive for elucidating the mechanisms by which other recognition molecules may guide cell interactions during development or modify their function as a result of injury, learning and memory.

Ankyrin-dependent and -independent mechanisms orchestrate axonal compartmentalization of L1 family members neurofascin and L1/neuron-glia cell adhesion molecule

Axonal initial segments (IS) and nodes of Ranvier are functionally important membrane subdomains in which the clustering of electrogenic channels enables action potential initiation and propagation. In addition, the initial segment contributes to neuronal polarity by serving as a diffusion barrier. To study the mechanisms of axonal compartmentalization, we focused on two L1 family of cell adhesion molecules (L1-CAMs) [L1/neuron-glia cell adhesion molecule (L1/NgCAM) and neurofascin (NF)] and two neuronal ankyrins (ankB and ankG). NF and ankG accumulate specifically at the initial segment, whereas L1/NgCAM and ankB are expressed along the entire lengths of axons. We find that L1/NgCAM and NF show distinct modes of steady-state accumulation during axon outgrowth in cultured hippocampal neurons. Despite their different steady-state localizations, both L1/NgCAM and NF show slow diffusion and low detergent extractability specifically in the initial segment but fast diffusion and high detergent extractability in the distal axon. We propose that L1-CAMs do not strongly bind ankB in the distal axon because of spatial regulation of ankyrin affinity by phosphorylation. NF, conversely, is initially enriched in an ankyrin-independent manner in the axon generally and accumulates progressively in the initial segment attributable to preferential binding to ankG. Our results suggest that NF and L1/NgCAM accumulate in the axon by an ankyrin-independent pathway, but retention at the IS requires ankyrin binding.

Association analysis of the NrCAM gene in autism and in subsets of families with severe obsessive–compulsive or self-stimulatory behaviors

OBJECTIVES

An autism susceptibility locus (AUTS1, MIM#608636) has been identified in chromosome 7q31. NrCAM is a candidate gene for AUTS1 because it is expressed in the brain and encodes a receptor involved in nervous system development. Polymorphisms in NrCAM have been reported to be associated with autism susceptibility and with substance abuse, implicating NrCAM in reward circuitry. Self-stimulatory, perseverative behavior in autism might be due to defects in reward circuitry. In addition, models of drug addiction have also borrowed from models of obsessive-compulsive behavior designed to reduce anxiety. Thus, our goals were to replicate previous associations of NrCAM with autism, making use of a large cohort, and to clarify whether NrCAM was associated with a specific endophenotype of autism in the repetitive behaviors and stereotyped interests domains.

METHODS

We genotyped six NrCAM single nucleotide polymorphisms in 352 families and we tested for association between these polymorphisms and autism in the entire cohort and in two subsets, one with severe obsessive-compulsive behaviors and one with pronounced self-stimulatory behaviors.

RESULTS

We found no association between single nucleotide polymorphisms of NrCAM and autism in our large cohort, or in the severe obsessive-compulsive behavior and self-stimulatory behavior subsets. However, we observed a significant overtransmission (21 transmitted vs 6 nontransmitted, chi2=12.054, P=0.0005) of the haplotype G-G-A-G-C-A of rs722519-rs1269622-rs405945-rs6958498-rs401433-rs439587 in the severe obsessive-compulsive behavior subset, likely driven by the G-C haplotype of rs6958498-rs401433, which itself showed significant overtransmission (31 transmitted vs 13 nontransmitted, chi2=8.844, P=0.003).

CONCLUSIONS

Overtransmission of particular haplotypes of NrCAM, that may relate to the expression level of NrCAM in the brain, appeared to be associated with autism in the severe obsessive-compulsive behavior subset.

Molecular genetics of addiction vulnerability

Classical genetic studies document strong complex genetic contributions to abuse of multiple addictive substances, to mnemonic processes that are likely to include those involved in substance dependence, and to the volumes of brain gray matter in regions that are likely to contribute to mnemonic/cognitive and to addictive processes. The working idea that these three heritable phenotypes are likely to share some of the same complex genetic underpinnings is presented. This review contains association-based molecular genetic studies of addiction that largely derive from my laboratory and their fit with linkage data from other laboratories. These combined results now identify many of the loci and genes that contain allelic variants that are likely to provide the heritable components of human addiction vulnerability. These data are also likely to have broad implications for neurotherapeutics. Drugs with potential abuse liabilities are widely used for indications that include pain, anxiety, sleep, seizure, and attentional disorders. There is increasing nonmedical use of these prescribed substances. Increasing information about addiction vulnerability gene variants should help to improve management of risks of dependence in individuals who receive such therapeutics. In addition, since mnemonic components that correlate well with individual differences in brain regional volumes are likely to play major roles in addiction processes, many addiction vulnerability genes are also good candidates to contribute to individual differences in mnemonic processes. Recently elucidation of addiction-associated haplotypes for the "cell adhesion" NrCAM gene illustrate several of these points.

A genetic study of the suppressors of the Engrailed-1 cerebellar phenotype

The mouse Engrailed genes, En1 and En2, play an important role in the development of the cerebellum from its inception at the mid/hindbrain boundary in early embryonic development through cell type specification events and beyond. In the absence of En1, the cerebellum and caudal midbrain fail to develop normally--a phenotype that we have previously reported to be strain dependent. On the 129/S1 strain background, En1 null alleles lead to mid/hindbrain failure, whereas on the C57BL/6 background, En1 deficiency is compatible with near normal cerebellar development. We have pursued this dramatic effect of genetic background by performing a genetic modifier screen through F1 backcross and F1 intercross matings. The backcross has yielded two strong candidate intervals with suggestive linkage to a third region. Moreover, variations in rescue frequency among subgroups within the backcross indicate gender and parent of origin influences on rescue penetrance. The intercross data reveal locus heterogeneity of the En1 modifiers, with more than one compliment of C57BL/6 and 129/S1 alleles capable of mediating the rescue phenotype. These findings highlight the complexity and plasticity of gene networks involved in brain development.

A role for Nr-CAM in the patterning of binocular visual pathways

Retinal ganglion cell (RGC) axons diverge within the optic chiasm to project to opposite sides of the brain. In mouse, contralateral RGCs are distributed throughout the retina, whereas ipsilateral RGCs are restricted to the ventrotemporal crescent (VTC). While repulsive guidance mechanisms play a major role in the formation of the ipsilateral projection, little is known about the contribution of growth-promoting interactions to the formation of binocular visual projections. Here, we show that the cell adhesion molecule Nr-CAM is expressed by RGCs that project contralaterally and is critical for the guidance of late-born RGCs within the VTC. Blocking Nr-CAM function causes an increase in the size of the ipsilateral projection and reduces neurite outgrowth on chiasm cells in an age- and region-specific manner. Finally, we demonstrate that EphB1/ephrin-B2-mediated repulsion and Nr-CAM-mediated attraction comprise distinct molecular programs that each contributes to the proper formation of binocular visual pathways.

The docking protein Cas links tyrosine phosphorylation signaling to elongation of cerebellar granule cell axons

Crk-associated substrate (Cas) is a tyrosine-phosphorylated docking protein that is indispensable for the regulation of the actin cytoskeletal organization and cell migration in fibroblasts. The function of Cas in neurons, however, is poorly understood. Here we report that Cas is dominantly enriched in the brain, especially the cerebellum, of postnatal mice. During cerebellar development, Cas is highly tyrosine phosphorylated and is concentrated in the neurites and growth cones of granule cells. Cas coimmunoprecipitates with Src family protein tyrosine kinases, Crk, and cell adhesion molecules and colocalizes with these proteins in granule cells. The axon extension of granule cells is inhibited by either RNA interference knockdown of Cas or overexpression of the Cas mutant lacking the YDxP motifs, which are tyrosine phosphorylated and thereby interact with Crk. These findings demonstrate that Cas acts as a key scaffold that links the proteins associated with tyrosine phosphorylation signaling pathways to the granule cell axon elongation.

The shed ectodomain of Nr-CAM stimulates cell proliferation and motility, and confers cell transformation

Nr-CAM, a cell-cell adhesion molecule of the immunoglobulin-like cell adhesion molecule family, known for its function in neuronal outgrowth and guidance, was recently identified as a target gene of beta-catenin signaling in human melanoma and colon carcinoma cells and tissue. Retrovirally mediated transduction of Nr-CAM into fibroblasts induces cell motility and tumorigenesis. We investigated the mechanisms by which Nr-CAM can confer properties related to tumor cell behavior and found that Nr-CAM expression in NIH3T3 cells protects cells from apoptosis in the absence of serum by constitutively activating the extracellular signal-regulated kinase and AKT signaling pathways. We detected a metalloprotease-mediated shedding of Nr-CAM into the culture medium of cells transfected with Nr-CAM, and of endogenous Nr-CAM in B16 melanoma cells. Conditioned medium and purified Nr-CAM-Fc fusion protein both enhanced cell motility, proliferation, and extracellular signal-regulated kinase and AKT activation. Moreover, Nr-CAM was found in complex with alpha4beta1 integrins in melanoma cells, indicating that it can mediate, in addition to homophilic cell-cell adhesion, heterophilic adhesion with extracellular matrix receptors. Suppression of Nr-CAM levels by small interfering RNA in B16 melanoma inhibited the adhesive and tumorigenic capacities of these cells. Stable expression of the Nr-CAM ectodomain in NIH3T3 cells conferred cell transformation and tumorigenesis in mice, suggesting that the metalloprotease-mediated shedding of Nr-CAM is a principal route for promoting oncogenesis by Nr-CAM.

A regulated switch of chick neurofascin isoforms modulates ligand recognition and neurite extension

Neural cell adhesion molecule neurofascin regulates the induction of neurite outgrowth, the establishment of synaptic connectivity and myelination. Neurofascin isoforms are generated by spatially and temporally controlled alternative splicing. Isoform NF166 is predominantly expressed in dorsal root ganglia from embryonal day 5 (E5) to E8, and a further neurofascin isoform NF185 appears at E9. Expression of neurofascin and its binding partner axonin-1 on sensory fibers implies functional interactions for neurite outgrowth. E7 sensory neurons require NF166-axonin-1 interactions for neurite extension, accordingly. The contribution of NF166-axonin-1 interaction for neurite outgrowth decreases in parallel with the appearance of NF185 on sensory neurons at E9. This finding may be explained by (1) alleviated intrinsic capability to use axonin-1 as a cellular receptor and (2) reduced binding of axonin-1 to NF185. Finally, NF166, but not NF185, serves as a cellular receptor for neurite induction via homophilic interactions with a neurofascin substrate.

Dual functional activity of semaphorin 3B is required for positioning the anterior commissure

Chemorepulsion by semaphorins plays a critical role during the development of neuronal projections. Although semaphorin-induced chemoattraction has been reported in vitro, the contribution of this activity to axon pathfinding is still unclear. Using genetic and culture models, we provide evidence that both attraction and repulsion by Sema3B, a secreted semaphorin, are critical for the positioning of a major brain commissural projection, the anterior commissure (AC). NrCAM, an immunoglobulin superfamily adhesion molecule of the L1 subfamily, associates with neuropilin-2 and is a component of a receptor complex for Sema3B and Sema3F. Finally, we show that activation of the FAK/Src signaling cascade distinguishes Sema3B-mediated attractive from repulsive axonal responses of neurons forming the AC, revealing a mechanism underlying the dual activity of this guidance cue.

Neurofascins Are Required to Establish Axonal Domains for Saltatory Conduction

Voltage-gated sodium channels are concentrated in myelinated nerves at the nodes of Ranvier flanked by paranodal axoglial junctions. Establishment of these essential nodal and paranodal domains is determined by myelin-forming glia, but the mechanisms are not clear. Here, we show that two isoforms of Neurofascin, Nfasc155 in glia and Nfasc186 in neurons, are required for the assembly of these specialized domains. In Neurofascin-null mice, neither paranodal adhesion junctions nor nodal complexes are formed. Transgenic expression of Nfasc155 in the myelinating glia of Nfasc-/- nerves rescues the axoglial adhesion complex by recruiting the axonal proteins Caspr and Contactin to the paranodes. However, in the absence of Nfasc186, sodium channels remain diffusely distributed along the axon. Our study shows that the two major Neurofascins play essential roles in assembling the nodal and paranodal domains of myelinated axons; therefore, they are essential for the transition to saltatory conduction in developing vertebrate nerves.

Mechanisms of axon ensheathment and myelin growth

The evolution of complex nervous systems in vertebrates has been accompanied by, and probably dependent on, the acquisition of the myelin sheath. Although there has been substantial progress in our understanding of the factors that determine glial cell fate, much less is known about the cellular mechanisms that determine how the myelin sheath is extended and stabilized around axons. This review highlights four crucial stages of myelination, namely, the selection of axons and initiation of cell-cell interactions between them and glial cells, the establishment of stable intercellular contact and assembly of the nodes of Ranvier, regulation of myelin thickness and, finally, longitudinal extension of myelin segments in response to the lengthening of axons during postnatal growth.

Synapse associated protein 102 is a novel binding partner to the cytoplasmic terminus of neurone-glial related cell adhesion molecule

Neurone glial-related cell adhesion molecule (NrCAM) is a member of the L1 family of transmembrane cell adhesion receptors which are involved in the development and function of the mammalian nervous system. How these receptors interact with intracellular signalling pathways is not understood. To date the only identified binding partner to the cytoplasmic terminus of NrCAM is ankyrin G. We screened a developing rat brain cDNA yeast two-hybrid library with the cytoplasmic domain of NrCAM to identify further intracellular binding partners. We identified synapse associated protein 102 (SAP102) as a new binding partner for NrCAM. The interaction was confirmed biochemically using glutathione S-transferase (GST)-pull-down and tandem affinity purification, and also immunocytochemically as NrCAM and SAP102 co-localized in COS-7 and cerebellar granule cells. Binding was specific to NrCAM as neither neurofascin nor L1 bound SAP102, and this interaction was reliant on the last three amino acids of NrCAM. Additionally, NrCAM constructs whose last three amino acids had been deleted appeared to have a dominant negative effect on neurite extension of cerebellar granule cells. This is the first interaction reported for NrCAM, and its association with SAP102 suggests that it is part of a larger complex which can interact with many different signalling pathways.

Accelerated Evolution of Nervous System Genes in the Origin of Homo sapiens

Human evolution is characterized by a dramatic increase in brain size and complexity. To probe its genetic basis, we examined the evolution of genes involved in diverse aspects of nervous system biology. We found that these genes display significantly higher rates of protein evolution in primates than in rodents. Importantly, this trend is most pronounced for the subset of genes implicated in nervous system development. Moreover, within primates, the acceleration of protein evolution is most prominent in the lineage leading from ancestral primates to humans. Thus, the remarkable phenotypic evolution of the human nervous system has a salient molecular correlate, i.e., accelerated evolution of the underlying genes, particularly those linked to nervous system development. In addition to uncovering broad evolutionary trends, our study also identified many candidate genes--most of which are implicated in regulating brain size and behavior--that might have played important roles in the evolution of the human brain.

Ankyrin-Based Subcellular Gradient of Neurofascin, an Immunoglobulin Family Protein, Directs GABAergic Innervation at Purkinje Axon Initial Segment

Distinct classes of GABAergic synapses are segregated into subcellular domains (i.e., dendrite, soma, and axon initial segment-AIS), thereby differentially regulating the input, integration, and output of principal neurons. In cerebellum, for example, basket interneurons make exquisitely precise "pinceau synapses" on AIS of Purkinje neurons, but the underlying mechanism is unknown. Using BAC transgenic reporter mice, we found that basket axons always contacted Purkinje soma before innervating AIS. This synapse targeting process followed the establishment of a subcellular gradient of neurofascin186 (NF186), an L1 family immunoglobulin cell adhesion molecule (L1CAM), along the Purkinje AIS-soma axis. This gradient was dependent on ankyrinG, an AIS-restricted membrane adaptor protein that recruits NF186. In the absence of neurofascin gradient, basket axons lost directional growth along Purkinje neurons and precisely followed NF186 to ectopic locations. Disruption of NF186-ankyrinG interactions at AIS reduced pinceau synapse formation. These results implicate ankyrin-based localization of L1CAMs in subcellular organization of GABAergic synapses.

Reduced GABAergic transmission and number of hippocampal perisomatic inhibitory synapses in juvenile mice deficient in the neural cell adhesion molecule L1

Cell adhesion molecules have been implicated in neural development and hippocampal synaptic plasticity. Here, we investigated the role of the neural cell adhesion molecule L1 in regulation of basal synaptic transmission and plasticity in the CA1 area of the hippocampus of juvenile mice. We show that theta-burst stimulation (TBS) and pairing of low-frequency presynaptic stimulation with depolarization of postsynaptic CA1 pyramidal cells induced similar levels of LTP in L1-deficient and wild-type mice. The basal excitatory synaptic transmission and density of asymmetric excitatory synapses in the stratum radiatum were also normal in L1-deficient mice. Since L1 is expressed not only by principal cells but also by inhibitory interneurons, we recorded inhibitory postsynaptic currents (IPSCs) evoked in CA1 pyramidal cells by minimal stimulation of perisomatic interneurons. L1-deficient mice showed a reduction in the mean amplitude of putative unitary IPSCs, higher values of the coefficient of amplitude variation, higher number of failures in transmitter release, and a reduction in frequency but not amplitude of miniature IPSCs. The use-dependent modulation of inhibitory transmission by paired-pulse or short tetanic stimulation was, however, normal in L1-deficient mice. The physiological abnormalities correlated with a strong reduction in the density of inhibitory active zones, indicating that L1 is involved in establishing inhibitory perisomatic synapses in the hippocampus.

NrCAM coupling to the cytoskeleton depends on multiple protein domains and partitioning into lipid rafts

NrCAM is a cell adhesion molecule of the L1 family that is implicated in the control of axonal growth. Adhesive contacts may promote advance of the growth cone by triggering the coupling of membrane receptors with the F-actin retrograde flow. We sought to understand the mechanisms leading to clutching the F-actin at the site of ligand-mediated clustering of NrCAM. Using optical tweezers and single particle tracking of beads coated with the ligand TAG-1, we analyzed the mobility of NrCAM-deletion mutants transfected in a neuroblastoma cell line. Deletion of the cytoplasmic tail did not prevent the coupling of NrCAM to the actin flow. An additional deletion of the FNIII domains to remove cis-interactions, was necessary to abolish the rearward movement of TAG-1 beads, which instead switched to a stationary behavior. Next, we showed that the actin-dependent retrograde movement of NrCAM required partitioning into lipid rafts as indicated by cholesterol depletion experiments using methyl-beta-cyclodextrin. Recruitment of the raft component caveolin-1 was induced at the adhesive contact between the cell surface and TAG-1 beads, indicating that enlarged rafts were generated. Photobleaching experiments showed that the lateral mobility of NrCAM increased with raft dispersion in these contact areas, further suggesting that TAG-1-coated beads induced the coalescence of lipid rafts. In conclusion, we propose that anchoring of NrCAM with the retrograde actin flow can be triggered by adhesive contacts via cooperative processes including interactions with the cytoplasmic tail, formation of cis-complex via the FNIII repeats, and lipid raft aggregation.

Regulation of pontine neurite morphology by target-derived signals

The molecular cues that regulate neurite morphology within the target environment are key to the formation of complex neural circuitry. During development of the ponto-cerebellar projection, pontine fibers sprout and form elaborate arbors within the inner cerebellar layer prior to arrival of their target cells, the cerebellar granule neurons. Here, we describe the biochemical fractionation of two granule neuron-derived factors that stimulate elaboration of pontine neurites. These factors were identified using a dissociated pontine bioassay and biochemically fractionated from granule cell (GC) conditioned medium (GCCM). One of the factors, STIM1, is a protein with a molecular weight greater than 30 kDa that is distinct from known neurotrophins. The other, STIM2, is a small, protease-resistant molecule with an estimated molecular weight below 1 kDa. We show that these factors stimulate pontine neurite elongation both independently and cooperatively and thus may contribute to the formation of elaborate pontine arbors within the cerebellar cortex.

Brain development in mice lacking L1-L1 homophilic adhesion

A new mouse line has been produced in which the sixth Ig domain of the L1 cell adhesion molecule has been deleted. Despite the rather large deletion, L1 expression is preserved at normal levels. In vitro experiments showed that L1–L1 homophilic binding was lost, along with L1-α5β1 integrin binding. However, L1–neurocan and L1–neuropilin binding were preserved and sema3a responses were intact. Surprisingly, many of the axon guidance defects present in the L1 knockout mice, such as abnormal corticospinal tract and corpus callosum, were not observed. Nonetheless, when backcrossed on the C57BL/6 strain, a severe hydrocephalus was observed and after several generations, became an embryonic lethal. These results imply that L1 binding to L1, TAG-1, or F3, and L1-α5β1 integrin binding are not essential for normal development of a variety of axon pathways, and suggest that L1–L1 homophilic binding is important in the production of X-linked hydrocephalus.

Ig superfamily cell adhesion molecules in the brain

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) were discovered 25 years ago based on their role in cell-cell adhesion. Ever since, they have played a major role in developmental neuroscience research. The elucidation of IgSF CAM structure and function has been tightly linked to the establishment of new areas of research. Over the years, our view of the role of the IgSF CAMs has changed. First, they were thought to provide "specific glue" segregating subtypes of cells in the nervous system. Soon it became clear that IgSF CAMs can do much more. The focus shifted from simple adhesion to CAM-associated signaling that was shown to be involved in the promotion of axon growth and the regulation of cell migration. From there it was a small step to axon guidance, a field that has been given a lot of attention during the last decade. More recently, the involvement of IgSF CAMs in synapse formation and maturation has been discovered, although this last step in the formation of neural circuits was thought to be the domain of other families of cell adhesion molecules, such as the neuroligins, the neurexins, and the cadherins. Certainly, the most striking discovery in the context of IgSF CAMs has been the diversity of signaling mechanisms that are associated with them. The versatility of signals and their complexity make IgSF CAMs a perfect tool for brain development.

The role of the ankyrin-binding protein NrCAM in node of Ranvier formation

Molecular events involved in Na+ channel clustering at the node of Ranvier have been investigated during early development. NrCAM, an ankyrinG-binding protein, precedes Na+ channels at cluster sites adjacent to the tips of Schwann cell processes. Both Na+ channel and ankyrinG sequestration at developing nodes are delayed in NrCAM null mutants. The action of NrCAM is manifest locally at individual nodes, rather than affecting overall neuronal expression, and is linked to glial interactions. During remyelination, Na+ channel clusters at new nodes are initially labile, and anchoring to the cytoskeleton appears to grow progressively with time. The distance between Na+ channel clusters across remyelinating Schwann cells (nascent internodes) increases markedly from 83 to 274 microm during node formation, arguing against schemes in which the loci of nodes are fixed in advance by the axon. A hypothesis for node formation in which axonal Na+ channels move by lateral diffusion from regions of Schwann cell contact, with clustering dependent on linkage to the cytoskeleton by ankyrinG, is proposed and tested in a computational model. To match experimental measurements, this latter reaction needs fast kinetics, and the early arrival of NrCAM is postulated to contribute to this requirement.

The role of the ankyrin-binding protein NrCAM in node of Ranvier formation

Molecular events involved in Na+ channel clustering at the node of Ranvier have been investigated during early development. NrCAM, an ankyrinG-binding protein, precedes Na+ channels at cluster sites adjacent to the tips of Schwann cell processes. Both Na+ channel and ankyrinG sequestration at developing nodes are delayed in NrCAM null mutants. The action of NrCAM is manifest locally at individual nodes, rather than affecting overall neuronal expression, and is linked to glial interactions. During remyelination, Na+ channel clusters at new nodes are initially labile, and anchoring to the cytoskeleton appears to grow progressively with time. The distance between Na+ channel clusters across remyelinating Schwann cells (nascent internodes) increases markedly from 83 to 274 microm during node formation, arguing against schemes in which the loci of nodes are fixed in advance by the axon. A hypothesis for node formation in which axonal Na+ channels move by lateral diffusion from regions of Schwann cell contact, with clustering dependent on linkage to the cytoskeleton by ankyrinG, is proposed and tested in a computational model. To match experimental measurements, this latter reaction needs fast kinetics, and the early arrival of NrCAM is postulated to contribute to this requirement.

Transgenic mice expressing F3/contactin from the TAG-1 promoter exhibit developmentally regulated changes in the differentiation of cerebellar neurons

F3/contactin (CNTN1) and TAG-1 (CNTN2) are closely related axonal glycoproteins that are differentially regulated during development. In the cerebellar cortex TAG-1 is expressed first as granule cell progenitors differentiate in the premigratory zone of the external germinal layer. However, as these cells begin radial migration, TAG-1 is replaced by F3/contactin. To address the significance of this differential regulation, we have generated transgenic mice in which F3/contactin expression is driven by TAG-1 gene regulatory sequences, which results in premature expression of F3/contactin in granule cells. These animals (TAG/F3 mice) display a developmentally regulated cerebellar phenotype in which the size of the cerebellum is markedly reduced during the first two postnatal weeks but subsequently recovers. This is due in part to a reduction in the number of granule cells, most evident in the external germinal layer at postnatal day 3 and in the inner granular layer between postnatal days 8 and 11. The reduction in granule cell number is accompanied by a decrease in precursor granule cell proliferation at postnatal day 3, followed by an increase in the number of cycling cells at postnatal day 8. In the same developmental window the size of the molecular layer is markedly reduced and Purkinje cell dendrites fail to elaborate normally. These data are consistent with a model in which deployment of F3/contactin on granule cells affects proliferation and differentiation of these neurons as well as the differentiation of their synaptic partners, the Purkinje cells. Together, these findings indicate that precise spatio-temporal regulation of TAG-1 and F3/contactin expression is critical for normal cerebellar morphogenesis.

Nr-CAM is a target gene of the beta-catenin/LEF-1 pathway in melanoma and colon cancer and its expression enhances motility and confers tumorigenesis

beta-catenin and plakoglobin (gamma-catenin) are homologous molecules involved in cell adhesion, linking cadherin receptors to the cytoskeleton. beta-catenin is also a key component of the Wnt pathway by being a coactivator of LEF/TCF transcription factors. To identify novel target genes induced by beta-catenin and/or plakoglobin, DNA microarray analysis was carried out with RNA from cells overexpressing either protein. This analysis revealed that Nr-CAM is the gene most extensively induced by both catenins. Overexpression of either beta-catenin or plakoglobin induced Nr-CAM in a variety of cell types and the LEF/TCF binding sites in the Nr-CAM promoter were required for its activation by catenins. Retroviral transduction of Nr-CAM into NIH3T3 cells stimulated cell growth, enhanced motility, induced transformation, and produced rapidly growing tumors in nude mice. Nr-CAM and LEF-1 expression was elevated in human colon cancer tissue and cell lines and in human malignant melanoma cell lines but not in melanocytes or normal colon tissue. Dominant negative LEF-1 decreased Nr-CAM expression and antibodies to Nr-CAM inhibited the motility of B16 melanoma cells. The results indicate that induction of Nr-CAM transcription by beta-catenin or plakoglobin plays a role in melanoma and colon cancer tumorigenesis, probably by promoting cell growth and motility.

Nr-CAM and neurofascin interactions regulate ankyrin G and sodium channel clustering at the node of Ranvier

Voltage-dependent sodium (Na(+)) channels are highly concentrated at nodes of Ranvier in myelinated axons and play a key role in promoting rapid and efficient conduction of action potentials by saltatory conduction. The molecular mechanisms that direct their localization to the node are not well understood but are believed to involve contact-dependent signals from myelinating Schwann cells and interactions of Na(+) channels with the cytoskeletal protein, ankyrin G. Two cell adhesion molecules (CAMs) expressed at the axon surface, Nr-CAM and neurofascin, are also linked to ankyrin G and accumulate at early stages of node formation, suggesting that they mediate contact-dependent Schwann cell signals to initiate node development. To examine the potential role of Nr-CAM in this process, we treated myelinating cocultures of DRG (dorsal root ganglion) neurons and Schwann cells with an Nr-CAM-Fc (Nr-Fc) fusion protein. Nr-Fc had no effect on initial axon-Schwann cell interactions, including Schwann cell proliferation, or on the extent of myelination, but it strikingly and specifically inhibited Na(+) channel and ankyrin G accumulation at the node. Nr-Fc bound directly to neurons and clustered and coprecipitated neurofascin expressed on axons. These results provide the first evidence that neurofascin plays a major role in the formation of nodes, possibly via interactions with Nr-CAM.